.\"
.\"  PCL by Davide Libenzi ( Portable Coroutine Library )
.\"  Copyright (C) 2003  Davide Libenzi
.\"
.\"  This program is free software; you can redistribute it and/or modify
.\"  it under the terms of the GNU General Public License as published by
.\"  the Free Software Foundation; either version 2 of the License, or
.\"  (at your option) any later version.
.\"
.\"  This program is distributed in the hope that it will be useful,
.\"  but WITHOUT ANY WARRANTY; without even the implied warranty of
.\"  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\"  GNU General Public License for more details.
.\"
.\"  You should have received a copy of the GNU General Public License
.\"  along with this program; if not, write to the Free Software
.\"  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
.\"
.\"  Davide Libenzi <davidel@xmailserver.org>
.\"
.\"  Original man page source by E.Toernig <froese@gmx.de>
.\"
.na
.TH PCL 3 "1.2" "GNU" "Portable Coroutine Library"
.SH NAME
co_create, co_call, co_resume, co_delete, co_exit_to, co_exit, co_current \- C coroutine management

.SH SYNOPSIS
.nf
.B #include <pcl.h>
.sp
.BI "coroutine_t co_create(void *" func ", void *" data ", void *" stack ", int " stacksize ");"
.nl
.BI "void co_delete(coroutine_t " co ");"
.nl
.BI "void co_call(coroutine_t " co ");"
.nl
.BI "void co_resume(void);"
.nl
.BI "void co_exit_to(coroutine_t " co ");"
.nl
.BI "void co_exit(void);"
.nl
.BI "coroutine_t co_current(void);"
.nl
.SH DESCRIPTION
The
.B Portable Coroutine Library (PCL)
implements the low level functionality for coroutines. For a definition
of the term
.B coroutine
see
.IR "The Art of Computer Programming" " by " "Donald E. Knuth" .
Coroutines are a very simple cooperative multitasking environment
where the switch from one task to another is done explicitly by a function call.
Coroutines are a lot faster than processes or threads switch, since
there is no OS kernel involvement for the operation. This document
defines an API for the low level handling of coroutines
i.e. creating and deleting coroutines and switching between them.
Higher level functionality (scheduler, etc.) is not covered.

.SS Functions
The following functions are defined:
.TP
.BI "coroutine_t co_create(void *" func ", void *" data ", void *" stack ", int " stacksize ");"

This function creates a new coroutine.
.I func
is the entry point of the coroutine.  It will be called with one
arg, a
.BR "void *" ,
which holds the data passed through the
.I data
parameter. If
.I func
terminates, the associated coroutine is deleted.
.I stack
is the base of the stack this coroutine will use and
.I stacksize
its size in bytes.  You may pass a
.B NULL
pointer for
.I stack
in which case the memory will be allocated by
.B co_create
itself.  Both,
.IR stack " and " stacksize
are aligned to system requirements.
A
.I stacksize
of less then 4096 bytes will be rejected.
You have to make sure, that the stack is large enough for your
coroutine and possible signal handlers (see below).  The stack
will not grow!  (Exception: the main coroutine uses the standard
system stack which may still grow) On success, a handle
.RB ( "coroutine_t" )
for a new coroutine is returned, otherwise
.BR NULL .

.TP
.BI "void co_delete(coroutine_t " co ");"

This function deletes the given coroutine
.IR co .
If the stack for this coroutine was allocated by
.B co_create
it will be freed.  After a coroutine handle was passed to
.B co_delete
it is invalid and may not be used any more.
It is invalid for a coroutine to delete itself with this
function.

.TP
.BI "void co_call(coroutine_t " co ");"

This function passes execution to the given coroutine
.IR co .
The first time the coroutine is executed, its entry point
.I func
is called, and the
.I data
parameter used during the call to
.B co_create
is passed to
.IR func .
The current coroutine is suspended until another one restarts it with a
.B co_call
or
.B co_resume
call. Calling oneself returns immediately.

.TP
.BI "void co_resume(void);"

This function passes execution back to the coroutine which either
initially started this one or restarted it after a prior
.BR co_resume .

.TP
.BI "void co_exit_to(coroutine_t " co ");"

This function does the same a
.B co_delete(co_current())
followed by a
.B co_call
would do.  That is, it deletes itself and then passes execution
to another coroutine
.IR co .

.TP
.BI "void co_exit(void);"

This function does the same a
.B co_delete(co_current())
followed by a
.B co_resume
would do.  That is, it deletes itself and then passes execution
back to the coroutine which either initially started this one or
restarted it after a prior
.BR co_resume .

.TP
.BI "coroutine_t co_current(void);"

This function returns the currently running coroutine.

.SS Notes
Some interactions with other parts of the system are covered here.
.TP
.B Signals
First, a signal handler is not defined to run in any specific
coroutine. The only way to leave the signal handler is
by a return statement.

Second, the signal handler may run with the stack of any coroutine,
even with the stack of library internal coroutines which have an
undefined stack size (just enough to perform a kernel call).
Using and alternate stack for signal processing (see
.BR sigaltstack (2))
is recommended!

Conclusion: avoid signals like a plague.  The only thing you may
do reliable is setting some global variables and return.
Simple kernel calls may work too, but nowadays it's pretty hairy
to tell, which function really is a kernel call.
(Btw, all this applies to normal C programs, too.  The coroutines
just add one more problem)
.TP
.BR setjmp / longjmp
The use of
.BR setjmp "(2)/" longjmp (2)
is limited to jumping inside one coroutine.  Never try to jump from
one coroutine to another with
.BR longjmp (2).

.SH DIAGNOSTICS
Some fatal errors are caught by the library.  If one occurs,
a short message is written to file descriptor 2 (stderr) and
a segmentation violation is generated.
.TP
.B [PCL]: Cannot delete itself
A coroutine has called
.B co_delete
with it's own handle.
.TP
.B [PCL]: Resume to deleted coroutine
A coroutine has deleted itself with
.BR co_exit " or " co_exit_to
and the coroutine that was activated by the exit tried a
.BR co_resume .
.TP
.B [PCL]: Stale coroutine called
Someone tried to active a coroutine that has already been
deleted.  This error is only detected, if the stack of the
deleted coroutine is still resident in memory.
.TP
.B [PCL]: Context switch failed
Low level error generated by the library in case a context switch
between two coroutines failes.

.SH SEE ALSO
Original
.B coroutine
library at
.BR http://www.goron.de/~froese/coro/coro.html " ."
GNU Pth library at
.BR http://www.gnu.org/software/pth/ " ."

.SH AUTHOR
Developed by Davide Libenzi <
.BR davidel@xmailserver.org " >."
Ideas and man page base source taken by the coroutine library developed by
E. Toernig <
.BR froese@gmx.de " >."
Also some code and ideas comes from the GNU Pth library available at
.BR http://www.gnu.org/software/pth/ " ."

.SH BUGS
There are no known bugs.  But, this library is still
in development even if it results very stable and pretty much ready for
production use.

Bug reports and comments to Davide Libenzi <
.BR davidel@xmailserver.org " >."

